Present modalities for treatment of malignant tumors include surgery, radiation therapy, chemotherapy, and immunotherapy which apply a physical or chemical force to alter the biological function of a tumor in order to affect its viability. Despite the medical advances that these modalities represent, most solid cancerous tumors carry with them a very poor prognosis for survival. Quality of life during and after treatment for survivors is often poor. The dismal prognosis for malignant solid tumors has led to continuing research for more effective treatment modalities with a lesser degree of disability and fewer side effects. In vitro and in vivo evidence indicates hyperthermia produces a significant anti-cancer activity through alteration of the physical environment of the tumor caused by increasing the temperature. Hyperthermia is more cytotoxic to tumor cells than normal cells because cancer cells are oxygen deprived, nutritionally deficient, and low in pH making them incapable of tolerating the stress imposed by elevated temperature. Tumor vasculature is immature, lacking the smooth muscle and vasoactivity which allows mature vessels to dilate, increasing blood flow to carry away heat, therefore intratumor temperatures exceed those in normal tissue. The mechanisms of selective cancer cell eradication by hyperthermia is not completely understood. However, four cellular effects of hyperthermia on cancerous tissue have been proposed: 1) changes in cell or nuclear membrane permeability or fluidity, 2) cytoplasmic lysomal disintegration, causing release of digestive enzymes, 3) protein thermal damage affecting cell respiration and the synthesis of DNA and RNA, and 4) potential excitation of immunologic systems.
The major forms of energy for generating hyperthermia to date include microwaves, radio frequency induction, radio frequency localized current, and ultrasound. Most of the techniques used to dispense these are non-invasive, i.e., the heat generating source is external to the body and does not invade the body. Consequently, the energy must pass through the skin surface and substantial power absorption by normal peripheral body tissue is unavoidable. Currently available external heating sources result in nonuniform temperature profiles throughout the tumor and increased temperature in normal tissue. It is desirable to selectively heat tissue deep in a patient's body, i.e., to heat the tumor mass without heating cutaneous and normal tissue.
Others have attempted the use of interstitial techniques to obtain local hyperthermia, with limited success. Interstitial heating of brain tumors through an implantable microwave antenna has been investigated. However, microwave probes are ineffective in producing precisely controlled heating of tumors. Temperatures may deviate as much as 10.degree. C from the desired target temperature. Besides, microwave activity adversely affects cellular structures and their integration, regardless of other thermal effects. The result is nonuniform temperatures throughout the tumor. Studies indicate that tumor mass reduction by hyperthermia is related to the thermal dose. Thermal dose is the minimum effective temperature applied throughout the tumor mass for a defined period of time. Hot spots and cold spots which occur with microwave hyperthermia may cause increased cell death at the hot spot, but ineffective treatment at cold spots resulting in future tumor growth. Such variations are a result of the microwave antenna's inability to evenly deposit energy throughout the tissue.
Since efferent blood flow is the major mechanism of heat loss for tumors being heated and blood flow varies throughout the tumor, more even heating of tumor issue is needed to ensure more effective treatment.
To be effective, the application and deposition of thermal energy to the tumor must be precisely controlled to compensate for the variations in blood flow. In addition, the therapy itself will perturb the tumor's vascular system during treatment causing variations in local perfusion around the probe. Thus, heat loss from a tumor will be time dependent and affected by the hyperthermia treatment. This demonstrates the need to both monitor and control the temperature of the tumor throughout treatment.
Benign Prostatic Hyperplasia (BPH) is a disease that is traditionally treatable by transurethral resection of the prostate (TURP). Patients who undergo a TURP are typically hospitalized for two to five days and convalesce afterward for another one to six weeks. Serious complications following TURP include failure to void or urinary retention in 10-15 percent of patients; bleeding that requires a transfusion in 5-10 percent of patients; urinary tract infection in 15-20 percent of patients; retrograde ejaculation in 60-75 percent of patients; and impotence in 5-10 percent of patients. As a result of the recovery time, medical costs, and likelihood of serious complications following a TURP, alternative methods for treating BPH have been attempted.
BPH has been treated by applying hyperthermia temperatures to the prostate of a patient. A hyperthermia device is inserted into the urethra so that the heat generating portion of the device is positioned in the prostatic urethra. To prevent damage to the internal and external sphincters, the heat generating portion of the device must not be in contact with or directed toward the sphincters. Damage to the internal sphincter results in retrograde ejaculation. Damage to the external sphincter results in incontinence. Damage to the nerves about the prostatic urethra results in impotence. Therefore, positively securing the proper position of the heat generating element is imperative for preserving these sphincters and their functions.
Several known catheters for use in the hyperthermia treatment of the prostate of a patient rely on microwave or radio frequency energy deposition for generating heat. One known catheter has a distally positioned bladder retention balloon, an inflatable prostate balloon, and a microwave antenna positioned in a longitudinal lumen of the catheter. The prostate balloon centers the antenna and compresses tissue while it is being irradiated for mitigating the problem of the microwave field intensity varying unevenly over the heated tissue.
Another known catheter has a distally positioned bladder retention balloon for limiting the proximal migration of the catheter. The bladder retention balloon also provides for maintaining the position of a diode centrally in the prostate for directing the peak of electromagnetic energy applied thereto by a microwave antenna toward the central area of the prostate.
Yet another known catheter has a distally positioned bladder retention balloon and a helical coil antenna for receiving electromagnetic energy from a microwave generator and heating tissue to hyperthermia temperatures in the range of 41.degree. to 47.degree. C.
One problem with each of these devices is that they use microwave or radio frequency energy deposition to effect heating. Radio frequency energy deposition resulting in heat generation is unpredictable due to the nonhomogeneous tissue between the applicator and grounding plate. Similarly, microwave energy deposition is unpredictable due to the different dielectric properties inherent in various types of tissue, such as muscle, fascia, and viscera. As a result, there can be uneven heating of anatomical regions with areas of overheated tissue and underheated tissue. The energy deposition heating technology can undesirably heat and damage the internal and external urethral sphincters. In addition, the use of energy deposition technology limits the size of the heat-emitting element. As a result, only limited modifications can be made to the catheter for tailoring the catheter to variations in individual patient anatomy.
Another problem with catheters using a distally positioned bladder retention balloon for limiting the proximal migration of the catheter is that the bladder retention balloon does not prevent a catheter from migrating distally. Since the longitudinal position of the catheter is not positively secured, the internal sphincter can be exposed to heat and damaged or destroyed.
An alternative to energy deposition technology for heating tissue is the application of thermally conducted heat. Several devices for applying heat directly to the rectum and gastrointestinal tract are known. For example, a thermoelectrical heat exchange capsule probe includes a plurality of thermocouples that get hot on one end and cold on the other when electrical current is passed therethrough. The probe can have a flexible, expandable sheath affixed to the outside thereof for containing a heat conducting fluid. The sheath is expandable for bringing a heated surface in contact with the tissue to be treated.
Another known device is a suppository appliance for the therapeutic treatment of hemorrhoids that is surrounded by a rigid, cylindrical jacket sized for intimately fitting in the anal canal of a patient. When electrical energy is applied to the appliance, a cylindrical electrical resistor generates heat inside the jacket to a predetermined maximum temperature of about 45.degree. C.
Yet another known device is a heatable dilation catheter for treating body tissue and including an elastic, expandable heat-emitting element, such as a braided stainless steel tube coated with silicone and mounted on a dilation balloon, for increasing the proximity of the heat-emitting element to tissue.
One problem with the use of any of these devices for treating BPH is that none of these devices can be affixed in a particular longitudinal position in a body passageway. As a result, anatomical structures that are preferably preserved can be exposed to high temperatures and damaged or destroyed. These devices are inappropriate for use in the urethra of a patient, wherein the internal and external urethral sphincters can be undesirably heated and damaged.